1. Field of the Invention
This invention relates to photopolymerizable compositions. More particularly, it relates to photopolymerizable compositions which show high sensitivity to a light source of a visible light range.
2. Description of the Prior Art
A number of image forming methods utilizing photopolymerizable systems are known. For instance, there is known a method of forming an image whereby a photopolymerizable composition which comprises a compound having an addition-polymerizable ethylenically unsaturated bond and a photopolymerization initiator, and optionally organic polymer binders, thermal polymerization inhibitors, colorants, plasticizers and the like is prepared; this photopolymerizable composition is applied onto a support either in the absence of any solvent or after dissolution in solvents, thereby forming a photosensitive material having a photopolymerizable composition layer thereon; and the photosensitive material is exposed to light in a desired image pattern, so that the exposed portions are polymerized and cured, and the remaining non-exposed portions are dissolved out and removed, thereby forming a cured relief image Alternatively, a photosensitive material is made by forming a photosensitive layer between two supports at least one of which is transparent; the photosensitive material is exposed in an imagewise pattern from the transparent support side, thereby causing the photosensitive layer to vary in adhesion strength; and thereafter the supports are separated from each other to form the intended image on the support. Still alternatively, there is a method of image formation which utilizes a change in toner adherence of a photopolymerizable composition layer on exposure to light.
The photopolymerization initiators for the photopolymerizable compositions used in the above methods are, for example, benzoin, benzoin alkyl ethers, benzophenone, anthraquinone, benzil, and Michler's ketone. However, these photopolymerization initiators have much lower photopolymerization initiating ability to a light source of the visible light region exceeding 400 nm than to a light source of the ultraviolet region below 400 nm. This places a great limit on the range of application of photopolymerizable composition comprising these initiators.
Several photopolymerization systems which are sensitive to visible light have been heretofore proposed In the past, it was described in U.S. Pat. No. 2,850,445 that certain kinds of photo-reducible dyes, e.g. Rose Bengal, Eosine, erythrosine, riboflavin and the like, had an effective visible light sensitivity. Since then, improvements have been reported including composite initiator systems comprising dyes and aliphatic amines (Japanese Patent Publication No. 20189/1969, systems of hexaarylbiimidazoles, radical generators, and dyes (Japanese Patent Publication No. 37377/1970, systems of hexaarylbiimidazoles and (p-dialkylaminobenzylidene) ketones (U.S. Pat. Nos. 3,652,275 and 4,162,162), systems of cyclic-cis-alphadicarbonyl compounds and dyes (U.S. Pat. No. 3,756,827), and systems of substituted triazines and merocyanine dye (Japanese Patent Application Laid-open No. 151024/1979). These systems are effective to visible light but are not satisfactory with regard to their photosensitive velocity. Thus, there is a continuing need for further improvements.
Some of the present inventors have already proposed photopolymerizable compositions which are highly sensitive to visible light (Japanese Patent Application Laid-open No. 21401/1982 and Japanese Patent Application Laid-open No. 19315/1983).